42nd Lunar and Planetary Science Conference (2011) 1053.pdf

DISTRIBUTION OF WINDBLOWN SEDIMENT IN SMALL CRATERS ON MARS: PRELIMINARY FIELD ANALOG STUDIES AT AMBOY CRATER, CALIFORNIA. R. L. Kienenberger1 and R. Greeley1, 1School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287 (Re- [email protected]).

Introduction: Shallow craters (<200 meters in diameter) or “hollows” [1] are prevalent near the Spirit rover landing site (Gusev Crater) and are thought to be mostly eroded secondary impact craters [2]. These craters contain asymmetric distributions of aeolian (windblown) sediments on the northwest side of the crater floor (Figure 1) indicating a prevailing wind direction either toward the northwest (depositing against the downwind side) or southeast (depositing on the upwind side). The study of wind streaks and bed- forms (ripples, dunes) can lead to a determination of local wind regimes on Mars [3]. However, the bed- forms which are common on the plains of Gusev Cra- ter are small (<50 meters) and cannot be used to infer wind direction because they have ambiguous shapes [4]. Therefore, a study of the location and characteris- tics of these asymmetric deposits with respect to crater morphometry is being conducted to determine the pre- dominant local wind direction during deposition. We Figure 2: (a) Google Earth image of part of Amboy present the preliminary results of field work at a terres- lava field, Mojave Desert, California with cinder cone trial analog site for comparison to HiRISE images and (arrow) and wind streak and enlargement (b) with topographic data from a digital elevation model [5] in endogenic craters 6, 7, 11 and 20 on a lava pressure Gusev Crater. plateau; 20 craters were initially identified for study Terrestrial Analog Background: A field area in but this research focused on the four craters noted. the Mojave Desert of California provides an analog for this area of Mars. The Amboy lava field (Figure 2) ovide a contrast to the basalt flow [7]. The predomi- consists of ~70 km2 of vesicular pahoehoe basalt with nant wind direction at Amboy is from northwest to an estimated age of 6,000 years [6]. A dark wind southeast, consistent with the wind streak in the lee of streak in the lee of a cinder cone extends ~4 km to the the cinder cone [8]. Several endogenic craters are lo- southeast. Alluvial deposits located upwind of the cated on the flow which provide a good analog for Amboy lava field provide a source for sand particles secondary impact craters on Mars because they have which are transported by wind to the lava field and pr- similar morphometric properties (depth, diameter, cir- cularity) and contain aeolian material. Methodology: Morphometric data (depths, diame- ters and circularities) for a total of 20 craters in the Amboy lava field were collected in the field and from aerial photographs. The rim-to-rim diameters of the craters were obtained by tracing the crater rim from an aerial photograph and overlaying circles of different sizes to obtain a best-fit circle for each crater following the method of [10]. The rim-to-floor depths were measured in the field using a laser level on the crater rim and a measuring rod located in the deepest part of the crater. Real-time wind measurements using suspended weighted styrofoam balls were collected at ~35 loca- tions in and around the two largest craters (Craters 6 Figure 1. MRO HiRISE image (PSP_001513_1655) and 7). The wind direction (azimuth) was collected showing a ~115m-in-diameter crater with asymmetric downwind to ensure the flow was not disrupted. All aeolian deposits near the Spirit rover landing site. measurements were taken within 30 minutes to obtain 42nd Lunar and Planetary Science Conference (2011) 1053.pdf

a “snapshot” of the wind flow around the crater. its in the northwest portions of three of the four cra- Sediment drift observations were made via dyed ters. The depth-to-diameter (d/D) ratios of these three sand placed within four craters in the plateau area craters are 0.049 (Crater 6), 0.057 (Crater 7) and 0.082 (Figure 2). Small conical piles of #30 sand (~300-600 (Crater 20). Crater 20 had nearly three times as many μm) were placed in seven locations within each crater. northwest-trending deposits (throughout the entire During subsequent visits to the field area, measure- crater floor) than the other two craters during the same ments were taken to document the change in each pile one month interval. The fourth crater, Crater 11, ex- over time. The lengths and orientations of all sediment hibited nearly all southeast-trending deposits with a drift patterns associated with dyed sand piles were d/D ratio of 0.019. Results of crater mapping indicate measured from the location of a numbered flag which Craters 6, 7 and 20 contain asymmetric deposits of was originally placed in the center of the pile. active aeolian material on the northwest sides of the Each crater was mapped to illustrate the locations craters (Figure 3) while Crater 11 contains active ae- of different zones and prominent features within the olian material throughout the entire crater floor. crater floor. Important areas included the active sand Conclusions: The results of real-time wind meas- deposits, lag deposits, transitional zones (from sandy urements and assessments of active sediment deposi- to lag surfaces), and desert pavement (Figure 3). Addi- tion with respect to the predominant wind direction tionally, topographic profiles for Craters 6 and 7 were (Figure 3) indicate reverse or stagnant wind flow oc- completed with a laser, measuring rod and measuring curs in three of the four craters with d/D ≥0.05. How- tape. These profiles were extended past the crater rims ever, reverse flow did not occur in the shallowest cra- to account for the slope of the surrounding terrain. An ter (d/D <0.02). Therefore, reverse wind flow is ex- azimuth of 130° was used for Crater 6 (Figure 3) to pected to cease within a d/D range of 0.02 to 0.05, correspond to the inferred predominant wind direction resulting in wind movement directly over the crater in the field area, as suggested by the wind streak in the floor in the downwind direction. Craters with asym- lee of the cinder cone. metric aeolian deposits near the Spirit landing site have d/D ratios of 0.034 to 0.076, indicating reverse flow may occur in these craters. Future Work: Wind tunnel experiments using crater models with known depths, diameters and rim heights (including a model of one analog crater from Amboy) will be completed to determine the areas of deposition within the craters and at what d/D ratio re- verse flow ceases. Finally, all results will be compared with HiRISE images and morphometric properties of craters on Mars to determine how wind patterns and the resulting aeolian deposition in and around small craters vary as a function of crater morphometry. Acknowledgments: This research was funded by the NASA Planetary Geology and Geophysics Pro- gram and the JPL Mars Exploration Rover project.

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